Violence in prison has dropped, with fewer assaults on inmates and guards.

Department of Corrections figures show along with a significant drop in the number of violent assaults over the past five years, fewer prisoners were requesting isolation for their own safety.

A revised policy for voluntary segregation was implemented in October last year with a new screening process to allow staff to consider individual prisoners' needs.

A snapshot of data from the same day in February in 2011 and 2012 showed a 6.3 percentage point drop in requests for segregation.

On February 10, 2011 there were 2248 prisoners on voluntary segregation out of a total prison population of 8470 prisoners. On that day this year, there were 1661 out of a total population of 8470.

Corrections general manager prison services Jeanette Burns says despite the drop, it is difficult to draw conclusions from the figures.

"The policy is due to be reviewed [this month], which will further examine the reasons for the decline in numbers," she said.

In the year to the end of January, 369 prisoners had been put into isolation, but only 296 of those were because of the danger they posed to others.

There were 15 prisoners in the past year who were isolated for medical reasons and 58 separated by officials because of risks to the prisoner's own safety.

Burns said the number of serious assaults on prison staff had also dropped dramatically since 1995.

"In 2010/11 there were 241 prisoner assaults on staff nationally. Serious assaults on prison staff have decreased almost 75 percent since the 1997/98 financial year, which had 43 recorded serious assaults. At the same time, the prisoner population has increased by 39 percent," she said.

In the same period, there were 862 prisoner on prisoner assaults, made up of 48 serious, 593 non serious and 221 no injury.

Burns said some incidents were unavoidable."Despite the efforts we make to ensure risks are mitigated, there will be some occasions where assaults occur and, on occasion, staff sustain injuries."

She said prisons could be "volatile environments... [and] while one assault is one too many, some prisoners have long histories of antisocial behaviour and resort to violence with little warning".The cost of keeping prisoners in isolation was not available, but Corrections said there was no significant cost differences between mainstream prisoners and those in segregation.

The average cost to house a prisoner was $249 per day, but the actual figures differed between men and women, and also depended on the security classifications they held.